1. Field of the Invention
The present invention relates to a low resistance value resistor suitable for use in applications such as current detector and the like, and relates in particular to a resistor made of a resistive alloy and having an electrode placed at each end of the resistor body.
2. Description of the Related Art
Low resistance value resistors of a plate- or ribbon-shape having an electrode placed at each end of a metallic base material are widely used in applications such as current detector and the like because of their characteristics of good heat dissipation and high current carrying capacity. Metallic materials serving as a resistor body include, for example, copper-nickel alloys, nichrome alloys, iron-chromium alloys and manganese alloys, and an electrode is placed at each end of the resistor. Conventional electrode structures are generally based on electroplated electrode on a metallic material mentioned above.
However, it is difficult to form a thick deposit on the resistor body by electroplating, and for this reason, uniformity of electric potential through the electrode is low, and the current path can not be stabilized, thereby making it difficult to manufacture low resistance value resistors of high precision. Also, bonding between the metallic material constituting the resistor body and the electrode produced by electroplating is weak, and problems occur when it is necessary to bend the resistor body for use, because the bond is susceptible to mechanical, thermal and electrical stresses.
Also, in some low resistance value resistors, instead of electroplated electrodes, electrodes are sometimes made by affixing a strip of copper or nickel to the resistor body by means of electron beam welding and the like. Even in such cases, such spot-type joining techniques produce small areas of contact through the attached strip, and similar problems of insufficient bonding strength and non-uniformity of current distribution are created. Therefore, problems are encountered in attaining high precision in low resistance value resistors, and obtaining low values of the temperature coefficient of resistance (TCR).
The present invention is provided in view of the background information described above and an object is to provide a low resistance value resistor that has a bonding strength sufficiently high for mechanical applications, a precise resistor value and superior characteristics of temperature coefficient of resistance (TCR).
The low resistance value resistor of the present invention is comprised by: a resistor body comprised by a resistive alloy; at least two electrodes, comprised by metal strips having a high electrical conductivity, formed separately on one surface of the resistor body; such that the metal strips are affixed on the resistor body by means of rolling and/or thermal diffusion bonding,
The low resistance value resistor is made by bonding metal strips on both ends of the resistor body having a high electrical conductivity by means of rolling and/or (thermal) diffusion bonding. In comparison with the electrodes made by electroplating or welding, the metal strip affixed by such rolling and/or diffusion bonding processes forms a diffusion layer at the interface of the metallic material of the resistor body or in the interior the resistor body. Therefore, because of the presence of the diffusion layer, the electrode are bonded strongly to the resistor body and a uniform distribution of current is obtained. The electrode structure thus produced is stable and is resistant to various stresses, including mechanical, thermal and electrical stresses.
Another aspect of the resistor is that a fused solder layer is formed on a surface of each electrode comprised by a metal strip.
Although the fused solder layer formed on the surface of the metal body is very thin, of the order of several micrometers, but the fused solder layer diffuses into the metallic material. For this reason, because of the presence of the fused solder layer diffusing into the interior of the metallic material, a high bonding strength is obtained and uniform current distribution is enabled. Therefore, as noted above, the electrode structure thus produced is stable and is resistant to various stresses, including mechanical, thermal and electrical stresses.
Still another aspect of the resistor is that the resistor body is trimmed by removing a portion of the body material along a direction of current flow to obtain a precisely controlled resistance value. Trimming to adjust a resistance value is performed by removing a portion of the body material in a thickness direction or along a corner section.
According to the present invention, a portion of the resistor body removed by a trimming process extends along the path of current flow so that the direction of the current flow in the trimmed resistor body is hardly affected by the removal of the portion. That is, as shown in FIG. 7 of the conventional low resistance value resistor, laser trimming is applied at right angles to the current flow to produce cutouts 1300, so that the direction of the current flow in the trimmed resistor is altered considerably, because the current must detour around the cutouts. Such a change in the current distribution created a problem that variations in the value of resistance are encountered in life testing and other tests. According to the present method of trimming, the resistance value is not changed in the life testing and other tests after the resistance trimming is performed. Because the current distribution is hardly affected and the current flows uniformly through the resistor body, thus there is no problem of variations in the resistance value of a trimmed resistor.